Filtration membranes are commonly used to separate fluid mixtures and solutions. For example, reverse osmosis (RO) and nanofiltration (NF) membranes are commonly used to remove salts, minerals and other dissolved ions in the desalination of seawater or brackish water, the production of dairy products, recovery of paint solids and other substances in metal finishing applications, and the like. Typical operating pressures for RO filtration systems range from 200-1200 psi. Similarly, the typical operating pressure range for NF systems is 75-150 psi.
Such membranes may be produced by coating a support material with layer of an aqueous casting solution and contacting a second solution layer to the aqueous casting solution layer to cause interfacial polymerization.
U.S. Pat. No. 4,277,344 to Cadotte (the xe2x80x9cCadotte referencexe2x80x9d) describes an aromatic polyamide membrane produced by the interfacial reaction of an aromatic polyamine with at least two primary amine substituents and an acyl halide having at least three acyl halide substituents. A porous support is coated with a layer of aqueous solution containing a monomeric aromatic polyamine reactant. The coated support is then contacted with a solution containing an amine-reactive polyfunctional acyl halide (preferably dissolved in a nonpolar organic liquid) and then dried. The Cadotte reference describes the use of trichlorotrifluoroethane (commonly known by the trade name xe2x80x9cFREONxe2x80x9d) as the organic solvent.
However, due to its environmentally adverse characteristics, FREON is no longer commercially available for this purpose. Using substitute solvents or alternative processes for making the membrane (such as contacting the polyamine reactant to the polyacyl halide in the vapor phase) produced membranes with inferior flux and salt rejection properties. The substitute solvents that have been tried to date are generally characterized by higher boiling points and low vapor pressures. Therefore, use of such solvents requires that the membranes be dried at higher temperatures, causing the membranes to lose the desirable flux and rejection characteristics.
U.S. Pat. No. 5,246,587 to Tomaschke describes an aromatic polyamide RO membrane that is made by first coating a porous support material with an aqueous solution containing a polyamine reactant and an amine salt. Examples of suitable polyamine reactants provided include aromatic primary diamines (such as, m-phenylenediamine or p-phenylenediamine or substituted derivatives thereof wherein the substituent is an alkyl group, an alkoxy group, a hydroxy alkyl group, a hydroxy group or a halogen atom); aromatic secondary diamines (such as, N,Nxe2x80x2-diphenylethylene diamine), cycloaliphatic primary diamines (such as, cyclohexane diamine); cycloaliphatic secondary diamines (such as, piperazine or trimethylene dipiperidine); and xylene diamines (such as m-xylene diamine). The support material is typically made of a polyarylether sulfone, such as a polysulfone and a polyether sulfone; a polyimide; or a polyvinylidene fluoride. The layer of aqueous amine solution is then contacted with an organic solvent solution containing a monomeric, aromatic, amine-reactive reactant, causing interfacial polymerization. The product is then dried to form a water permeable membrane.
U.S. Pat. No. 6,245,234 to Koo et al. describes a composite polyamide RO membrane that is made by first coating a porous polysulfone support with an aqueous solution containing: 1) a polyfunctional primary or secondary amine; 2) a polyfunctional tertiary amine; and 3) a polar solvent. The excess aqueous solution is removed and the coated support is then dipped in an organic solvent solution of trimesoyl chloride (TMC) and a mixture of alkanes having from eight to twelve carbon atoms. The resulting composite membrane is then rinsed in a 0.2% sodium carbonate (Na2CO3) aqueous solution.
U.S. Pat. No. 6,177,011 to Hachisuka et al. describes a RO membrane comprising a sponge layer and a separation layer formed on the sponge layer. The separation layer either contains or is coated with an electrically-neutral organic substance or polymer, such that the surface zeta potential of the layer is with +/xe2x88x920.10 millivolts at pH 6.
U.S. Pat. No. 6,183,640 to Wang describes a polymeric membrane having permanent, internal anionic charges. The membrane is cast on a porous support structure from a solution containing a sulfone polymer, an anionic charge-modifying agent, a nonsolvent and a solvent.
However, none of these membranes has the superior flux and salt rejection characteristics of the present invention. Additionally, several of the processes for membrane manufacturing described above consume large quantities of expensive chemicals, making the end product unaffordable in many popular commercial applications.